TABLE 3.

Growth rates and nitrate reductase activities of mutant strains derived from Synechococcus sp. strain PCC 7942

Strain	Relevant genotype	Growth rate (days−1)a		Nitrate reductase (mU/mg of protein)b
		Ammonium	Nitrate
PCC 7942	Wild type	2.76	3.07	36.5
CSLM26	moaA::lacZ-C.K3	2.10	<0.01	<0.1
CSLM27	moeA::lacZ-C.K3	1.61	<0.01	<0.1
CSLM32	moeA::C.S3	2.90	<0.01	<0.1
CSLM34	moaC::C.S3	2.06	<0.01	<0.1
CSLM35	moaC::lacZ-C.K3	3.26	<0.01	<0.1
CSLM37	moaC::C.S3, moaA::lacZ-C.K3	1.80	<0.01	<0.1
FM6c	moaE::Tn901	1.80	<0.01	<0.1

^aAmmonium-grown cells of the wild-type and mutant strains were washed twice with medium lacking combined nitrogen and used to inoculate, at a final concentration of 0.2 μg of chlorophyll/ml, cultures with the indicated nitrogen source. The cultures were bubbled with air-CO₂ (98:2) at 30°C in the light for 24 h and then diluted eightfold with fresh culture medium with the same nitrogen source. To estimate the growth rate, protein content was measured in aliquots sampled periodically from the last set of cultures. 

^bFor nitrate reductase activity determinations, ammonium-grown cells of the wild-type and mutant strains were washed twice with medium lacking combined nitrogen and incubated for 6 h in medium containing nitrate as the sole nitrogen source, bubbled with air-CO₂ (98:2) at 30°C in the light. Nitrate reductase activity was then determined in aliquots of each culture. 

^cAlthough the phenotype of this strain has been previously reported (26), it is included in this experiment for the sake of comparison.